The present invention relates to a non-linear optical coupler and to a parametric oscillator incorporating such a coupler. As a result of electrical controls, such a coupler is able to transfer between two waveguides having a common coupling zone, all or part of the light energy carried by one of them. It also makes it possible to modulate the light energy carried by one of the waveguides.
The term integrated optics is used to define thin layer monolithic structures used for processing light signals obtained by deposition, diffusion and etching by masking, as used in the production of integrated electronic circuits As a result of these processes, it is possible to produce linear structures characterized by a higher refractive index than that of the surrounding medium, whilst forming waveguides along which the light is propagated by a sequence of total reflections or progressive refractions.
It is known to link two such waveguides, by arranging them parallel to one another over part of their path in order to obtain directional couplers. As a result of the evanescent wave phenomenon, the energy carried in the first guide passes progressively into the second guide and a maximum of the transferred energy is observed at the end of a certain length, called the coupling length, which is dependent on the geometrical and optical parameters of the structure and in particular the value of the refractive indices of the materials constituting the two guides and the medium separating them, then the energy progressively passes back from the second guide into the first guide and so on. It is also known that by using an electro-optical material as one of the materials constituting the guides or the medium separating them to vary the index under the action of an electrical field which, by acting on the coupling length, makes it possible to electrically control the energy portion transferred from one guide to the other. On the basis of this principle, it is also possible to obtain a light modulator by arranging, parallel to the guide carrying the light wave, a guide section into which is transferred a varyingly large portion of said energy.
In addition, there are frequency converters for producing, from guided electromagnetic radiation of frequency .omega., guided electromagnetic radiation, whose frequency is a multiple of the frequency .omega.. These converters are more particularly used in the field of integrated optics, a term used by analogy with integrated electronic circuits, which are monolithic structures using thin layers.
It is known that the propagation of an electromagnetic wave of frequency .omega. in an anisotropic material, such as birefringent crystals of potassium deuterophosphate (KDP), gives rise to harmonic waves of frequency p..omega. (p integer, p.gtoreq.2). However, most of the emergent energy remains at the fundamentally frequency .omega., which indicates that the conversion efficiency is generally mediocre. Such a non-linear phenomenon also occurs during the refraction, as well as the reflection accompanying it, of a light beam on an interface separating two media having different refractive indices. However, the intensity of the conversion phenomenon is well below that of the preceding case.
In the field of integrated optics, a problem arises with respect to the use of this non-linear phenomenon with a view to obtaining an adequate conversion efficiency, because the deposition of anisotropic material in thin layers cannot be carried out by conventional methods used for producing electronic integrated circuits.